During semiconductor wafer processing, semiconductor devices are structures are defined and formed in the wafer using well known patterning and etching processes. In many of these processes, a photoresist (PR) material is used to assist in the formation of these structures. Commonly, photoresist material is deposited on the wafer and then exposed to light filtered by a mask reticle to achieve a patterned photoresist mask. The reticle is generally a glass plate that is patterned with exemplary feature geometries that block light from propagating through the reticle.
After passing through the reticle, the light contacts the surface of the photoresist material. The light changes the chemical composition of the photoresist material such that a developer can remove a portion of the photoresist material. In the case of positive photoresist materials, the exposed regions are removed, and in the case of negative photoresist materials, the unexposed regions are removed. Thereafter, the wafer is etched to remove the underlying material from the areas that are no longer protected by the photoresist material, and thereby define the desired features in the wafer.
The minimum feature size of integrated circuits (ICs) continues to shrink with each generation of semiconductor wafer processing improvements. As transistors and metal lines get smaller and move closer together, this puts increasing demands on the photoresist materials and patterns. Previously insignificant third-order variables now play a major role in IC design and fabrication.
One significant limitation in the existing state of the art pertains to the thickness or vertical height of the photoresist layer. In existing technologies, depth of focus limitations inherent in the patterning equipment prevent the application of photoresist in layers thicker than they are currently used. However, there is a need in the industry for ever thicker mask layers to achieve certain fabrication structures. Currently, this need is met by transferring the mask pattern from the photoresist pattern to an underlying film to create an underlying “hard mask” having taller features which are then used to form various structures on the substrate. This approach has the unfortunate drawback of requiring additional steps to form the second mask which requires more time and has its own unique complications.
One example of a prior art process is depicted in FIGS. 1A-1D. In FIG. 1A a substrate 100 is provided having a number of structural and possibly circuit features. In this depiction, a layer 101 that is to be patterned is shown. The prior art achieves a narrow feature spacing in the following manner. The process continues with the formation of a hard mask layer that is to be etched into the desired hard mask pattern. FIG. 1B depicts layer 101 with an etch stop layer 102 formed thereon. Further, a hard mask layer 103 is formed on the etch stop layer 102. It is over this hard mask layer 103 which a photoresist layer is formed, exposed, and developed into mask pattern 104. The process continues as shown by FIG. 1C. As shown trenches 105 are etched into the hard mask layer 103 to define a plurality of tall and narrow hard mask features 103a that now serve as a hard mask pattern 106. This pattern 106 defines a pattern to be etched into the underlying material 101 to define a desired structure. This can enable high density feature formation and robust resistance to etch conditions. If desired, the photoresist material of the mask 104 can be removed prior to patterning the underlying substrate 101.
In a subsequent substrate etch step the pattern is transferred into the substrate 101 using an etch process to achieve the desired pattern in the layer 101. FIG. 1D depicts a series of etched features 101a formed in the layer 101. Typically, once the pattern 101a has been transferred to the substrate 101, the hard mask pattern of features 103a are removed so that further processing can occur.
The process of generating the hard mask 106 is time consuming and many of the processes used for its formation and removal can degrade the substrate and the quality of the pattern therein. Also, this process is limited to the critical dimension that is established by the initial mask pattern.
Methods for reducing process steps and processes that eliminate the need for a hard mask are advantageous. Moreover, processes capable of forming pattern doubled structures are needed.